Up to now, the powder magnetic core using soft magnetic metal particles is being developed as the magnetic core for the motor, the actuator, the generator, the reactor, the choke coil and the like. Generally speaking, it is known that the magnetic core prepared by compressing the powder has a low mechanical strength. A high mechanical strength is required in the magnetic core so that the damage of the product can be prevented in use. Further, as the electromagnetic component is downsized, its working frequency tends to be high. If the working frequency becomes higher, the eddy current loss inside the magnetic core will sharply rise. Thus, a powder magnetic core with a high electrical resistivity is required to prevent the occurrence of such a loss.
In a preparation method which is used to improve the mechanical strength of the powder core, it is proposed to increase the molding pressure or the temperature during the thermal treatment. However, although the powder core prepared by such treatments has an increased mechanical strength, the insulating coating film formed on the surface of the soft magnetic metal particles is likely to peel off or decompose during the molding process or the thermal treatment, and thus the electrical resistivity becomes low. If the electrical resistivity decreases, the core loss increases as the eddy current inside the magnetic core rises, leading to the decrease of the output or the efficiency of the product.
Thickening the insulating coating film covering the soft magnetic metal particles is known to be effective in order to prepare a power magnetic core with a high electrical resistivity. However, if the insulating coating film is thickened, the strength will become lower as the stress is likely to concentrate on the interior of the coating film. Therefore, there is no soft magnetic powder core with both a high electrical resistivity and a high strength.
In order to solve the technical problem, for example. Patent Document 1 has disclosed a preparation method in which the solution formed by dissolving a complex of elements which constitute the vanadium oxide based glass with a low melting point or dissolving an alkoxide in an organic solvent is coated on the surface of iron powders and then the coating film is dried and subjected to a thermal treatment to form a vanadium oxide based glass coating film with a low melting point. In addition, Patent Document 2 has disclosed a preparation method in which an oxide coating film containing Mg is formed on soft magnetic metal powders to obtain a soft magnetic composite material with a high mechanical strength. Also, Patent Document 3 has disclosed a soft magnetic composite material with a high mechanical strength and a high specific resistance and a method for preparing the same, wherein the soft magnetic composite material is obtained by mixing insulating coated soft magnetic particles and glass powders with a low melting point and then firing the mixture.